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Oxidation properties of ”Solar Salt”Wassila Benaissa, Douglas Carson

To cite this version:Wassila Benaissa, Douglas Carson. Oxidation properties of ”Solar Salt”. AIChE Spring Meeting2011 & 7. Global Congress on Process Safety (GCPS), Mar 2011, Chicago, United States. pp.NC.�ineris-00976226�

Oxidation properties of "Solar Salt"

Wassila BENAISSAINERI S

Parc Technologique Alata BP 2, F-60550 Verneuil-en-HalatteWassila.Benaissa@ineris.fr

Douglas CARSONINERI S

Keywords: solar salt, oxidation.

Abstract

Solar Salt is a name sometimes given to a molten salt mixture made up of about 60% of sodiumnitrate (NaNCh) and 40% of potassium nitrate (KNO3). This composition is near the eutecticpoint and is thermally stable until 600°C. It is popular in Industrial Solar Energy Projects and isused for storing energy in the form of heat to smooth out the peaks in electricity production.However for some technologies, combustible substances, like a thermal fluid for example, maycome into contact with the molten salt. The aim of the paper is then to study the oxidizingproperties of the solar salt in order to estimate the energy released by a potential reactionbetween the salt and combustibles and estimate safety issues. In that purpose, four types ofexperiments were carried out: the standard UN O.I test for solid oxidizers, the standard UN 0.2test for liquid oxidizers, differential scanning calorimetry (DSC) and isothermal calorimetry(C80). The experimental program demonstrates the oxidizing properties of the solar salt andshows that the reactivity of solar salt with other combustibles has to be taken into account in aglobal risk analysis of a Solar Energy Central.

1. Introductio n

Regarding the potentially dramatic effects of climate change and the estimated diminution ofavailable fossil fuels, the development of new renewable energies have became a priority inresearch and development in order to anticipate the energy supply solutions of the future. Solarpower, allowing conversion of sunlight into electricity, can be seen as one of the most promisingways [1]. In particular, during the last decades, concentrating solar power (CSP) has shown veryinteresting potentialities, through different sustainable power generation applications at industryscale [2]. Four different technology approaches are currently used for concentrating solar power:sterling dish, linear reflector, parabolic trough and power tower. In each case, the heat sourceprovided by the sunlight is captured then concentrated by mirrors. One of the largest advantagesof CSP against other sources of renewable energy such as photovoltaic or wind is the capabilityto provide a more flexible repartition of the power: to do so, the solar energy can be stored inthermal reservoirs and released during periods of peak power demand, cloudy weather or even atnight [3]. Storage technologies can be either "direct" or "indirect". Indirect means that thestorage medium is not heated directly by the concentrators. Indirect systems use a heat transferfluid instead, typically synthetic oil, which passes through a heat exchanger with the storagemedium to heat it indirectly. Typically the transfer fluid is synthetic oil and the storage mediumis molten salts, generally nitrates. A practical example of this technology named Andasol 1 andimplemented in Spain in 2008, use cool tanks (about 290°C) and hot tanks (about 390°C) ofmolten salts, with about 29,000 tones in each tank. In this configuration the storage capacity isabout 1100 MWh that means about 7.5 h of full-load production of electricity and a veryinteresting benefit in terms of power supply duration [4].

In the field of safety, one can ask the risks of storing such a quantity of chemical product on asame spot. Indeed, combustible substances, like a thermal fluid for example, may come intocontact with the molten salt. The aim of the paper is to study the oxidizing properties of a solarsalt in order to estimate the energy released by a potential reaction between the salt andcombustibles and estimate safety issues. The molten salt chosen as a case study is a mixturemade up of about 60% of sodium nitrate (NaNOs) and 40% of potassium nitrate (KNO3). Fourtypes of experiments were carried out: the standard UN O.I test for solid oxidizers, the standardUN O.2 test for liquid oxidizers, differential scanning calorimetry (DSC) and isothermalcalorimetry (C80).

2. Literatur e overview for the oxidizing properties

2.1 Pure nitrates

As the solar salt is a mixture of potassium and sodium nitrate, the reactivity of these two pureproducts has to be analyzed to understand the behavior of the salt.Potassium nitrate is an ionic salt, odorless and melting at 333°C. It is stable at ambienttemperature and pressure and can safely be stored during a long period if protected frommoisture. When heated, potassium nitrate produces oxygen (cf. Eq.l). This reaction isendothermic and reversible.

KNO3 <-> KN02 + V2 O2 [Eq. 1]

Ullmann's Encyclopedia [5] gives a decomposition temperature equal to 530°C but smallervalues around 400°C have also been published [6, 7]. According to the Wiley Guide to ChemicalIncompatibilities [8], potassium nitrate is noncombustible, but enhances the combustibility oroxidation rate of many materials and chemical reactions can cause fire and explosion.

Sodium nitrate is an ionic salt, odorless and melting at 308°C. It is stable at ambient temperatureand pressure and can be safely stored during a long period if protected from moisture. Whenheated, potassium nitrate produces oxygen (cf. Eq.l). This reaction is endothermic andreversible.

NaNO3 <-> NaNO2 + lA O2 [Eq. 2]

Ullmann's Encyclopedia [5] gives a decomposition temperature around 500°C but smaller valuesaround 380°C have also been published [6, 7]. According to Wiley' Guide to ChemicalIncompatibilities [8], sodium nitrate is non combustible, but enhances the combustibility oroxidation rate of many materials and chemical reactions can cause fire and explosion.

2.2 Classification

Potassium nitrate and sodium nitrate are classified 5.1 (oxidizing substances) by the"Recommendations on the Transport of Dangerous Goods" [9] and class 1 oxidizers by the U.S.Hazardous Material Code, NFPA 400, Chapter 15, Oxidizer Solids and Liquids [10]. The lastclassifies oxidizer materials according to their ability to cause spontaneous combustion and howmuch they can increase the burning rate. Of four classes, class 1 is the least dangerous and refersto substances which "slightly increase the burning rate of combustible materials" but "do notcause spontaneous ignition when they come in contact with them".The sodium nitrate and potassium nitrate mixture is also clearly identified in the classification5.1 and UN number 1499.

2.3 Solar salt

Solar salt is composed of two powerful oxidizers, both stable until 400°C. The mixture of thesetwo salts without impurity gives a eutectic which melts at 219°C. From this point, the solar saltbecomes a transparent liquid which can be used as a thermal fluid. Some industrial applicationsshows that the operating temperature of a solar central can reach 400°C for the hot storage, for aquantity more than 20,000 tones. Regarding safety, one can ask two questions: is the solar saltthermally stable and what are its oxidizing properties?

3. Experiments and results

3.1 Experimental approach

Two calorimetric devices and two tests from the Transport of Dangerous Goods Manual [11]have been used: differential scanning calorimetry (DSC), Calvet calorimeter, test O.I foroxidizing solids and test O.2 for oxidizing liquids.

The calorimetric experiments are used to study the thermal stability of the solar salt and thereaction with combustibles.One important constraint was met for the UN standards: the tests and criteria are significantlydifferent for solid and liquid oxidizers. Depending on the temperature, the solar salt state can beone or another. Some modification was then made to manage the two states.

3.2 Chemicals

High purity (99.99%) sodium nitrate (NaNCh) and potassium nitrate (KNO3) were purchasedseparately from MERCK. At ambient temperature, these two products are generally white crystalpowders. The solar salt is then prepared by mixing these two powders. The solar salt was groundto less than 100 |im and tested as such.A combustible oil containing biphenyl has been chosen to represent the heat transfer fluid.The characteristics of the cellulose used to perform the tests follow the recommendation of themanual of tests and criteria published by the United Nation for Transport of Dangerous Goods[11]. It is dried fibrous cellulose, with a fiber length between 50 and 250 |im and a meandiameter of 25 |im. It is dried in a layer no more than 25 mm thick at 105°C for 4 hours and keptin a desiccator until cool and needed for use. The water content is less than 0.5% by dry mass.Pure potassium bromate has been used as reference for the 0.1 test.

3.3 DSC experiments

The DSC is a micro-calorimetric technique which measures a heat flux difference between a fewmilligrams sample and a reference, placed in a same temperature control oven and following thesame scan rate (cf. picture 1). It is used to detect all kind of exothermic and endothermicbehavior: phase transitions, heat capacity, chemical reactions/decompositions, etc. The test givesaccess in thermal power and energy released by the sample [12].

Pictur e 1. DSC calorimeter

Crucibles

Oven

Each few milligrams sample is introduced at ambient temperature in a stainless steel crucible.The crucible is then sealed and put in an oven with an empty crucible as reference. The twocrucibles are heated at 5 K/min from ambient temperature up to 700°C. The heat flux and theenergy released are then measured (see figures 1 to 7). Table 1 summarizes the results.

Table 1. DSC test on Solar Salt, cellulose and thermal fluid

DSCn°l

DSC n°2

DSC n°3

DSC n°4

DSC n°5

DSC n°6

DSC n°7

Component A

solar salt

solar salt

solar salt

solar salt

potassium

chloride

100 %

50%

85%

45%

60%

Component B

cellulose

cellulose

thermal fluid

thermal fluid

thermal fluid

thermal fluid

100 %

50%

100 %

15%

55%

40%

Tonset(°C)

stable

300

240

610

550

540

stable

Enthalpy(J/g)

-588

-3054

-545

-707

-2454

Enthalpy(J/g

combustible)

-588

-6108

-545

-4799

-4490

6 -

5 -

3 4 -

tx 3 -cIS 2

1 -

0 -

-1 -

(

Figure 1: DSC n°1 -100% solar salt

Heat ramp: 5 K/min

le

Enthalpy: 146 (J/g)

VV) 100 200 300 400 500 600 700

Temperature (°C)

6

5 •

3 4 •

t1 3£ 2

1 -

o •

- 1 •

Figure 2: DSC n°2 -100% cellulose

Heat ramp: 5 K/min

Enthalpy:-588 (J/g)

1I1l11...

v•* •— - ,

•^ - ^— • —

) 100 200 300 400 500 600 700Temperature (°C)

7

6

5"S

X

0)

-2

Figure 3: DSC n°3 - 50 % solar salt + 50 % cellulose

Heat ramp : 5 K/min

/

—

1

11J....

\\\

Enthalpy:-3054 (J/g)

100 200 300 400 500Temperature(°C)

600 700 S00

hsI 2

Figure 4: DSC n°4 -100% thermal fluid

Heat ramp: 5 K/min

Enthalpy:-426 (J/g)

/ \\

100 200 300 400Temperature (°C)

500 600 700

5 •

4 -

I3X35= T

n 2

a

1 -

0 •

-1 -

(

Figure 5: DSC nc5 - 85 % solar salt + 15 % thermal fluid

Heat ramp: 5 K/min

melting

fif

Enthalpy:-707 (J/g)

—J—\

) 100 200 300 400 500 600Temperature (°C)

10

3

is

-2

Figure 6: DSC n°6 - 45 % solar salt + 55 % thermal fluid

Heat ramp: 5 K/min

meltin9

Enthalpy: -2454 (J/g)

/

J

1

1

1100 200 300 400

Temperature(°C )500 600 700

3) 6

iX

é0}

Figure 7: DSC n°7 - 60 % potassium chloride + 40 % thermal fluid

Heat ramp: 5 </min

100 200 300 400 500

Temperatur e (°C)500 TOO

On figure n°l, the endothermic peak is characteristic of the melting point of the solar salt and itis thermally stable up to 650°C (cf. figure 1).The cellulose decomposes at 300°C with an enthalpy of-588 J/g (cf. figure 2). When solar salt ismixed to cellulose, we observe a significant difference: the onset temperature is much less andthe enthalpy is almost ten times larger. It seems that the solar salt enhances the decomposition ofthe cellulose (cf. figure 3).

We can reach a similar conclusion for the thermal fluid (cf. figure 4). The last is very stable until545°C. When mixed with solar salt, the initial temperature decrease around 500°C and theenthalpy is also almost ten times larger (cf. figure 5 and 6). The figure 7 confirms that withanother inert product, no thermal activity is detected.These results illustrate the influence of solar salt acting like a typical oxidizer i.e. it increases thereactivity of a combustible substance when both are thoroughly mixed.

3.4 C80 experimentsThe Calvet calorimeter is also a differential calorimeter that may be operated isothermally or inthe scanning mode as a DSC. It allows working with larger sample volumes (few grams) andshows a higher sensitivity (around 0,1 W.kg"1) essentially due to the measurement principal,based on a array of thermocouples totally surrounding the cells (sample and reference) (cf.picture 2). As in the DSC, the test gives thermal power and energy released by the sample andallows detecting exothermic activity [13].

Picture 2. C80 calorimeter

Oven

Reference cell

Two tests were carried out. Firstly, the cellulose is put in a closed cell and tested in DS mode at0.5 K/min from ambient temperature to 300°C (cf. figure 8). Secondly a cell constructed of twocompartments separated by a membrane is used: 300 mg of cellulose is introduced in the first

compartment and 300 mg of solar salt is introduced in the second. During a first step, the twoproducts are heated separately until 300°C. When the heat flux is stabilized, the membrane isbroken and the two products mixed (cf. figure 9). When the mixing occurs, a highly exothermicreaction can be observed that is not detected when the cellulose is pure.

0,15

? 0,1 •

X

I

0

-0,05 •

(

Figure 8: C80 n°1 -100 % cellulose

- ?:

Heat ramp: 0.5 K/min

, •• •

/

~<

7

/

•••• •

I

If

ta y

Enthalpy : -332 (J/g)

- 300

- 250

- 200 1CB

|

<B

- 100 Ô

- 50

] 100 200 300 400 500 600 700 800 900 1000 1100 1200Time (min)

0 2 j -

0,15 - -

0,1 - -

3S, 0,05 - -XCISI 0 L

-0,05 - -

J

0

Figure 9: C80 nc2 - 50 % solar salt + 50 % cellulose

ds

f-

7

•:

ce

/

>m

/

ulpcs

-,Hf

Sit

\

1

or

\

/f

/

\

/

\

^^

hie atr

Melting ateutectic

anip 0.5 hï/m n

>

/

M x ng

\

-

Enthalpy:-1152 (J/g)

//

/

•̂ .

/

r 350

• 300

• 250

- 200 _,

•a(B

• 150 1

CD

•1 0 0 - 2

- 50

250 500 750 1000 1250 1500 1750 2000Time (min)

3.5 O.l test

The UN manual defines the 0.1 test as a method to measure the potential for a solid substance toincrease the burning rate or burning intensity of a combustible substance when the two arethoroughly mixed (cf. picture 3). For the 0.2 test, a liquid is also considered as an oxidizer if themixture made with the combustible substance can spontaneously ignite.

Picture 3. UN O.I disposal

0.1 tests have been conducted on the solar salt mixed with dry fibrous cellulose in mixing ratioof 1:1 and 4:1, by mass of sample to cellulose. The burning characteristics of the mixture arecompared with the standard 3:7 mixture (around 100 s), by mass of potassium bromate tocellulose. If the burning time is equal to or less than this standard mixture, the burning timesshould be compared with those from the packing group I or II reference standards, 2:3 (around54 s) and 3:2 (around 4 s) ratios, by mass of potassium bromate to cellulose respectively. Theresults are summarized in table 1.

Table 2. UN O.I test on Solar Salt

Solar Salt

Reference

mixtures

Classificationlimit s

Packing Group

I limit

Packing Group

II limit

Packing Group

II I limit

Mixtur e

24 g solar salt /

6 g cellulose

15 g solar salt/15 g cellulose

18 g potassium bromate /

12 g cellulose

12 g potassium bromate /

18 g cellulose

9 g potassium bromate /

21 g cellulose

Molarrati o

4:1

1:1

3:2

2:3

3:7

Reaction time (s)

N°l

7

20

6

63

115

N°2

8

19

5

69

112

N°3

7

20

6

59

118

N°4

9

21

6

65

123

N°5

8

19

7

63

120

Average

7.8

19.8

6.0

63.8

117.6

The solar salt as tested is assigned to the class 5 (oxidizing substances and organic compounds),division 5.1 (oxidizing substances) and packaging group II (substances presenting mediumdanger), borderline packaging group I (substances presenting high danger). According to this testand criteria of the United Nation Manual, the solar salt has to be considered as a strong oxidizer.

4.1 0.2 test

According to the 0.2 tests procedure, the liquid to be tested is mixed to 1:1 ratio, by mass, withfibrous cellulose, poured into a pressure vessel with an exposed electric heating coil. The mixtureheated (by action of the heating coil) in a pressure vessel and the rate of pressure rise iscompared with those of reference substances. Since solar salt is a powder mixture at ambienttemperature, the procedure had to be slightly adjusted:

• the pressure vessel was placed empty in an oven at 250°C overnight,

• a mixture of 2.5 g of solar salt with 2.5 g of cellulose was then prepared in a glass

container.

• the pressure vessel was filled with the mixture to cover the ignition coil,

• the pressure was vessel was covered with aluminum foil and placed in the oven for 30 moreminutes,

• the pressure vessel was taken out of the oven and closed according to conventionalprocedure,

• the heating coil was energized.

To complete the approach, two other mixtures were tested in the exact same conditions: amixture of solar salt and an inert powder, a mixture of solar salt and an inert liquid. Since theoperating temperature is much higher than the conventional one, it is important to verify that thecellulose thermal behavior doesn't influence the results. Table 2 summarized the pressure riseobtained. Perchloric acid solution (55%) has been chosen as a typical reference for a solutionclassified 5.1 (oxidizing substances) and packaging group I: the mean pressure rise time for a 1:1mixture with cellulose is 59 ms.

Table 3. UN O.2 test on Solar SaltComponent A

Perchloric acid (55%)

Solar salt

Solar salt

Inert mineral oil

Component B

cellulose

cellulose

Inert powder

cellulose

Test n°l Test n°2

59 ms (typical time taken from the UNmanual. Solution 5.1 packaging group I)

23 ms

No significantpressure rise

No significantpressure rise

30 ms

No significantpressure rise

No significantpressure rise

Since the tests have not been carried out in the exact same condition as the one described in theUN manual, it is not possible to directly extrapolate the classification of the molten solar salt.However, the result shows a significant reaction when the solar salt and the combustible aremixed showing a potential influence of the salt on the combustion.

4. Conclusion

Despite the good knowledge of the hazard characteristics of the pure components of the solarsalt, the oxidizing characteristics of the mixture were not clearly identified. The experimentalprogram has demonstrated the oxidizing properties of the solar salt and shows that there is apotential important reactivity of solar salt with other combustibles. Since it is one of the technicaloptions used in the solar centrals using concentrated solar power, this property has to be takeninto account in a global risk analysis of a Solar Energy Central.

5. References

[I ] Weisz, P.B., "Future energy supply society - Challenges in evaluation criteria andinterdisciplinary research", Vacuum, 80, 2006

[2] SolarPACES, Greenpeace International and ESTELA, "Concentrating Solar Power -Why Renewable Energy is Hot", Global Outlook 2009, available athttp://www.solareuromed.com/v2/download/concentrating-solar-power-2009.pdf

[3] Skumanich, A., "CSP: Developments in heat transfer and storage materials", RenewableEnergy Focus, Volume 11, Issue 5, September-October 2010

[4] Medrano, M., Gil, A., Martorell, I , Potau, X. and Cabeza, L.F., "State of the art on high-temperature thermal energy storage for power generation, Part 2 - Case Studies",Renewable and Sustainable Energy Reviews, Volume 14, Issu 1, January 2010

[5] "Ullmann's Encyclopedia of Industrial Chemistry"

[6] "Hazards in the Chemical Laboratory", Edited by S.G. Luxon, Fifth edition, 1992

[7] Kramer, CM., Munir, Z.A. and Volponi, J.V, "Differential scanning calorimetry ofsodium and potassium nitrates and nitrites", Thermochimica Ada, 1982, 55.

[8] Pohanish, R.P. and Greene, S.A., "Wiley Guide to Chemical Incompatibilities", JohnWiley and Sons Publication, Second edition, 2003

[9] United Nations, "Recommendations on the Transport of dangerous Goods - ModelRegulations", Fifteenth revised edition, 2007

[10] National Fire Protection Association (NFPA), Hazardous Material Code, NFPA 400,National Fire Protection Association, Quincy, MA USA, latest edition

[I I ] United Nations, "Recommendations on the Transport of dangerous Goods - Manual ofTests and Criteria", Fourth revised edition, 2008

[12] Hôhne, G., Hemminger, W. and Flammerschein, H.-J., "Differential ScanningCalorimetry and Introduction for Practitioners", Springer, Berlin.

[13] Calvet, E. and Prat, H., "Microcalorimetrie: applications physic-chimiques etbiologiques", Masson, Paris.